1. In a large bowl, mix ground sirloin, garlic salt, black pepper and olive oil until incorporated. Separate into 6 equal portions, form into patties. Grill on med-high heat 7 min. each side for medium done burgers.
2. While the patties are cooking, in a small bowl, mix mayonaise, ketchup, mustard and pickle until blended. This will be your spread.
3. When burger has 1 minute left on the grill, place cheddar cheese on top of patty.
4. Place buns on grill to lightly toast.
5. Pull buns off the grill, spread mayonaise mix on both sides of buns, layer lettuce, onion, burger patty with cheese, tomato and avocado.